Some sublimated type color thermal transfer printers use an ink sheet coated with a yellow (Y) ink area, a magenta (M) ink area, and a cyan (C) ink area extending sequentially in a longitudinal direction, and also uses a rolled paper as a recording paper. In such a thermal transfer printer, heat is applied from a thermal head to the ink sheet and an image of each color is printed overlappedly in the same area of the recording paper to form a color image.
In this case, when the image area formed has an upper limit which is the ink areas and an image smaller than the ink areas is printed, for example, when a single L size (3R size) image is printed by using an ink sheet for 2L size (5R size), a problem is that because only a half of each of the Y, M, and C color ink areas is used and therefore each remaining ink area is not used, the ink use efficiency is bad.
To solve this problem, as a printing method which improves the ink use efficiency, a method of rewinding an ink sheet after producing a print of an L size image by using a front half ink area on an ink sheet take-up side of each of Y, M, and C color ink areas of the ink sheet, and printing the next L size image by using a rear half ink area on an ink sheet supply side which is a yet-to-be-used area in each of the ink areas which have been used for the previous print, thereby producing a print of two screens from the single set of Y, M, and C ink areas is known.
In this case, the ink sheet is rewound temporarily after the thermal head is detached from the ink sheet, and the thermal head is brought into contact with the ink sheet again. At this time, a problem is that because the ink sheet which has been used for a print of the 1st of the two screens is placed immediately before the print start point of the 2nd of the two screens, and the used ink sheet receives damage due to the heat energy applied thereto from the thermal head, an appropriate ink sheet tension cannot be applied to the print start point of the 2nd of the two screens, and hence wrinkles can easily occur in the ink sheet. To solve this problem, a conventional method of printing the 1st of two screens by using an ink sheet supply side of the ink sheet and then printing the 2nd of the two screens from an ink sheet take-up side of the ink sheet, thereby preventing wrinkles from occurring in the print start point of the 2nd of the two screens, has been provided (for example, refer to patent reference 1).
Further, another conventional method of predicting the degree of damage which an ink sheet will receive after printing the 1st of two screens on the basis of the density of a whole image which is the target for the print of the 1st of the two screens, and the average grayscale of the whole image, and determining whether ink areas on an ink sheet take-up side or on an ink sheet supply side of the ink sheet are used for the print of the 1st of the two screens has been provided (for example, refer to patent reference 2).